ABSTRACT The innate immune system plays critical roles in maintaining a healthy lung and in shaping the responses to challenge. As for most biological processes, the extent, pattern, duration, and consequence of inflammation are controlled by a balance between positive and negative factors. Our preliminary data indicates that stromelysin-2 (MMP10), a member of the matrix metalloproteinase gene family, functions to govern the influx and activation state of infiltrated macrophages and has diverse roles in cigarette smoke-induced lung damage. On one hand, it moderates the extent of inflammation, particularly, the influx and activation state of infiltrated macrophages. On the other, MMP10 promotes alveolar destruction. We propose that these phenotypes are due to differences in macrophage activation. Specifically, we hypothesize that MMP10 moderates macrophage activation towards an M1 phenotype (classically activated; proinflammatory), thereby promoting the differentiation of more M2 (alternatively activated or anti-inflammatory) macrophages. M2 macrophages, which are known to over-represented in the lungs of smokers, are considered to be reparative and to mediate matrix remodeling. However, with sustained inflammation, a reparative phenotype would lead excessive remodeling and tissue destruction. Hence, we further hypothesize that by promoting the differentiation of M2 macrophages, MMP10 contributes-directly or indirectly-to development of emphysema. To study the role of MMP10 in alveolar destruction more detail, we propose to 1) determine the role of MMP10 in shaping macrophage differentiation.; 2) determine the relative roles of epithelial- and macrophage-derived MMP10; and 3) assess the role of MMP10 in governing the macrophage response to bacterial infection.